Recently, substrates of image pickup devices such as CCD and CIS are often made of an epitaxial silicon wafer including an epitaxial layer provided by a vapor deposition on a silicon wafer. It is crucially important for such an epitaxial silicon wafer for an image pickup device that an amount of heavy-metal impurities in the silicon wafer is lowered. This is because, when heavy-metal impurities are present in a wafer, so-called white defects (defects in device characteristics, which are unique to an image pickup device) are caused.
During the production of epitaxial silicon wafers using vapor deposition, H2 and Si material gases are used for the vapor deposition of the epitaxial layer. By-products are also generated during the vapor deposition, which are deposited in a chamber. The deposited by-products are a source of contamination. Accordingly, in order to remove the by-products, the chamber is regularly cleaned. Hydrogen chloride gas is used as a cleaning gas.
Even when a highly anti-corrosive metal is used for a component of a vapor deposition apparatus, since hydrogen chloride gas is highly corrosive, the component of the vapor deposition apparatus still corrodes in an atmosphere of highly-concentrated hydrogen chloride gas. Then, metal contaminant (e.g. metal chloride) caused by the corrosion is introduced into the wafer and, consequently, the produced epitaxial silicon wafer is contaminated with the metal.
In order to reduce the metal contamination caused by the vapor deposition apparatus, it has been proposed to cover a part of vapor deposition apparatus components, which is made of a material containing metal and is to be in contact with the gas, with a non-metal protection film and to use an O-ring not containing Ti at a connecting portion of each of the components (see Patent Literature 1: JP-A-2010-135388). According to Patent Literature 1, since a contact of gas and metal can be prevented with the use of the above-described vapor deposition apparatus, the metal contamination can be avoided. Consequently, it is reported that a high-quality epitaxial silicon wafer with small number of white defects, total concentration of four elements of Mo, W, V and Nb is 4×1010/cm3 or less, and Ti concentration of 3×1012/cm3 or less can be produced.
However, according to the method disclosed in the above Patent Literature 1, since it is necessary to regularly renew the coating of the component(s) coated with the non-metal protection film, complicated work has to be done each time the regular re-coating is done. Further, it is expected to be difficult in terms of the design to completely cover an entirety of the metal component(s) that is to be in contact with gas using a protection film.